Important properties of hydrocarbon fluids are the distillation information generally determined by ASTM D-86, ASTM D-1078, or the ASTM D-1160 vacuum distillation technique for heavier materials, flash point, density, Aniline Point as determined by ASTM D-611, aromatic content, as determined for example by UV spectroscopy, Bromine Index, as determined for example by ASTM D2710, viscosity, colour and refractive index. Fluids are classified as paraffinic such as the Norpar® fluids marketed by ExxonMobil chemical Company, isoparaffinic such as the Isopar® fluids marketed by ExxonMobil Chemical Company; dearomatized fluids such as the Exxsol D® fluids marketed by ExxonMobil Chemical Company; naphthenic materials such as the Nappar® fluids marketed by ExxonMobil Chemical Company; non-dearomatised materials such as the Varsol® fluids marketed by ExxonMobil Chemical Company and the aromatic fluids such as the Solvesso™ heavy aromatic fluids marketed by ExxonMobil Chemical Company.
As with any hydrocarbon product whose starting point is crude oil, the degree of purity which may be achieved in a hydrocarbon fluid grade or “cut” covers a wide range from relatively crude to relatively pure. Typically, industrial-scale production of hydrocarbon fluids results in a product having a boiling range generally covering at least about 5° C. (e.g., hexane) and extended up to close to 100° C. (e.g, kerosene). As used herein, the term “boiling range” means the temperature spread between the initial temperature at which the specified cut boils and the dry point temperature.
The chemical nature and composition of hydrocarbon fluids varies considerably according to the use to which the fluid is to be put. Although each grade of hydrocarbon fluid has commercial use, there are special applications which require hydrocarbon fluids of exceptional purity with respect to aromatics and/or heteroatoms, particularly sulfur and oxygen-containing species, but not ultra-high purity with respect to isomers and/or carbon number of the hydrocarbons themselves. For instance, in the case of hexane produced and consumed on an industrial-scale, it may typically be sold in a grade which begins boiling at about 64° C. or 65° C. and finishes boiling at about 70° C., and which contains a wide variety of hydrocarbons in addition to n-hexane. This type of grade is effective for many processes, e.g., as a processing aid in the manufacture of polymers, as a solvent in a solvent extraction process, and the like (as opposed to ultra-high purity spectroscopic-grade n-hexane available on a relatively small laboratory-scale quantities from, for instance, Aldrich Chemicals, which may contain 95% or higher n-hexane).
Currently available dearomatized fluids having a distillation cut of from 50° C. to 350° C. are available on industrial scale through fractionation of kerosene, diesel or other petroleum cuts, followed by one or several hydrogenation processes using hydrogenation catalysts, typically, nickel or nickel-based catalysts. As used herein, the term “distillation cut” means that the material identified has an initial boiling point greater than or equal to the lower temperature (e.g., here 50° C.) specified and a dry point less than or equal to the higher temperature specified (e.g., here 350° C.). As used herein, the term “actual cut” when applied to a temperature range identifies exactly the initial boiling and dry point of the material identified. Thus, using the previous hexane example, there is a hexane grade which may be described as an actual cut from 64° C. to 70° C. within the distillation cut of 50-350° C. and having a boiling range of 6° C.
The levels of aromatics achieved by the aforementioned fractionation and/or hydrogenation methods vary depending on the feed that is hydrogenated, the higher boiling hydrocarbons being much more difficult to dearomatize than the lower boiling range hydrocarbons. Typically, aromatic levels of from 100 ppm to 8000 ppm can be achieved by these methods, for hydrocarbon fluids having a distillation cut of from 150° C. to 300° C., and a boiling range of less than 40° C. Lower aromatic levels may be achieved for compositions having an initial boiling point below 150° C., but for such low boiling hydrocarbons, it would be desireable for certain applications to reduce further the level of heteroatom-containing molecules. There is thus a need to find methods for the preparation of hydrocarbon fluids having amounts of aromatics and/or amounts of heteroatom-containing molecules, such as sulfur-containing and oxygen-containing molecules, that are lower than those achievable by the present industrial scale dearomatization methods.
Dearomatized hydrocarbon fluids, such as those based on kerosene, diesel, or other refinery feeds, are used in a variety of end uses including inks, consumer products, metal rolling, water treatment, coatings, drilling muds, agricultural formulations, and the like. Historically, dearomatized fluids contained about 1-2 wt. % aromatic and other unsaturated species (e.g., olefins). By way of example, a petroleum feed may be hydrogenated over a catalyst such as nickel, Ni/Mo, Ni/Mo/W to provide an intermediate product having, for instance, 20 wt. % aromatics, followed by a finishing step, which comprises hydrogenation over a catalyst such as nickel. This level of aromatics is unsatisfactory for end uses such as water treatment. In addition to levels of aromatics and olefinic species on the order of 1-2 wt. %, other typical impurities in the final product include high levels of heteroatoms such as sulfur compounds, nitrogen compounds, and oxygenates. These other impurites are detrimental when dearomatized hydrocarbon fluids are used, for instance, as solvents in catalytic processes or in processes requiring an ultra low level of such impurities, such as in semiconductor processing. The use of Ni in hydrogenation reactors is also a safety concern because of the danger of runaway reactions.
High pressure hydrogenation methods can achieve aromatic levels on the order of 100-500 ppm. However, the investment in such methods is quite high, and create increased safety concerns. Furthermore, presently available hydrogenation methods typically do not decrease oxygenate content in the final product, which is a drawback for many reasons, such as increased catalyst deactivation in processes using the hydrocarbon fluid as process fluid.
Accordingly, a method of producing ultra low levels of aromatics, oxygenates, and other impurities in hydrocarbon fluids without such high pressure methods is highly sought after. In addition, aromatic content even lower than 100 ppm is desired because of increased environmental concerns and increased regulatory requirements concerning aromatic content in hydrocarbon fluids used in consumer products, water treatment methods, and the like.
The prior art has not provided a solution to all these problems in an economical manner and/or environmentally sound manner.
Numerous patents teach dearomatization by adsorption, such as U.S. Pat. Nos. 4,567,315 and 5,220,099, but adsorption processes are both environmentally unsound and energy inefficient solutions.
U.S. Pat. No. 4,795,840 is an example of a hydrogenation process using a pressure on the order of 30-100 kg/cm2 (about 30-100 atm). The product, however, retains at least 1 wt. % of alkyl tetralins, an aromatic species. Subsequent to hydrogenation, a separation using molecular sieves is applied.
U.S. Pat. No. 5,151,172 teach hydrogenating a hydrocarbon feedstream using a catalyst comprising Pt/Pd on mordenite achieving, according to examples presented, as low as 16 wt. % aromatics content.
U.S. Pat. No. 5,830,345 teaches a gasoline blend made by a reaction involving simultaneous hydrogenation and isomerization of a benzene-enriched reformate stream using a dual catalyst comprising an hydrogenation catalyst and a zeolite catalyst having pores of about 5 Å.
U.S. Pat. No. 5,831,139 teach selectively upgrading naphtha to a more aliphatic gasoline having low aromatics by a process comprising selective isoparaffin synthesis from heavy naphtha and a recycle stream which is subject to ring cleavage, the overall effect being that the molecular weight and boiling point of the hydrocarbons are reduced.
U.S. Pat. No. 5,855,767 teach a high pressure (≧30 bar) hydrocarbon conversion process comprising contacting a cracking catalyst including a zeolite-beta as a first component, a second component which may be MCM-41, and a hydrogenation component.
U.S. Pat. No. 5,855,767 teach a process for saturation of lube range hydrocarbons using a nobel metal on zeolite inorganic oxide support under conditions of a temperature range of 350-700° F., 150-3500 psig using a feed having a viscosity of 50-600 SUS at 100° F. Aromatic content is reduced to as low at 3 vol. % according to patentee.
U.S. Pat. No. 5,993,644 teaches a process for producing a lubricating oil basestock comprising steps of hydrotreating, dewaxing, and hydrogenation. According to the examples, the process can achieve aromatic content as low as about 6 wt. %; similar results for aromatic levels are shown in U.S. Pat. No. 6,399,845, which teaches the manufacture of diesel fuel from middle distillate with a catalyst that both removes aromatics and isomerizes paraffins.
U.S. Pat. No. 6,030,921 teach hydrogenation of lubricating oil stocks in a process involving hydrocracking and hydrogenation. The examples in the patent show conversion of aromatics of about 86%.
U.S. Pat. No. 6,207,870 and U.S. Pat. No. 6,541,417 teach hydrogenating aromatics in gas oil cut at pressures of about 6 MPa using a silicon-doped catalyst.
U.S. Pat. No. 6,306,289 teach a method of hydrotreating a hydrocarbon oil using a catalyst comprising a Group VIII metal and “a large amount” of silica. Examples show that at 60 kg/cm2 sulfur content may be reduced to about 500 ppm. Results for aromatics levels are not provided.
U.S. Pat. No. 6,509,510 concerns a process for hydrogenating an aromatic polymer using a silica- or alumina-supported Group VIII catalyst having a pore size of at least 100 Å.
U.S. Pat. No. 6,541,417 utilizes a silicon-doped Group VIII catalyst for hydrogenation of hydrocarbon feeds, particularly dearomatization of gas oil cuts.
U.S. 2001/0013484 and 2002/0117425 are directed to achieving low polyaromatic hydrocarbons (PAH). Examples show reduction of PAH to an amount of above 9 wt. %.
U.S. 2003/0188991 teaches a mesoporous silica catalyst capable of hydrogenation, isomerization, hydrocracking and numerous other reactions. Pd on MCM-41 is used for comparison purposes (see, e.g., Table 15 of the patent).
U.S. 2004/0181103 teaches a supported catalyst useful in dearomatizing fuels. According to the examples, aromatic levels as low as 480 ppm are achieved.
WO 01/14501 discusses reducing the concentration of aromatics and/or olefins in a diesel fuel using a catalyst comprising Pt/Pd on MCM-41, with “complete aromatics saturation” at temperatures greater than 450° F. (about 232° C.)
WO 2004/024319 teaches a catalyst for selectively upgrading paraffinic feedstock to isoparaffin products useful for blending with gasoline.
EP 0 698 073 relates to a process for the hydrogenation of aromatics in hydrocarbonaceous feedstocks, the examples showing a reduction in aromatics content to just below about 1 wt. %.
Other patents of interest include U.S. Pat. Nos. 5,612,422; 5,853,566; 6,084,140; 6,136,181; 6,197,721; 6,264,826; 6,280,608; 6,281,397; 6,417,287; 6,432,297; 6,579,444; and 2003/0173252.
The present inventors have surprisingly discovered a method of hydrogenating hydrocarbon fluids that does not require the use of high pressure systems and provides for ultra low levels of impurities, particularly aromatics and other unsaturates.